Electroplating power supply apparatus

ABSTRACT

A power supply apparatus for use in electroplating includes an input-side rectifier ( 32 ). A rectifier output is converted into high-frequency signals in inverters ( 38   a,    38   b ), which are transformed in transformers ( 48   a   , 48   b ). When the transformed high-frequency signals from the transformers are of positive polarity, they are rectified by a diode ( 50   a  or  50   b ) to cause a positive current to be supplied to a load ( 60 ). When the transformed high-frequency signals are of negative polarity, they are rectified by a diode ( 50   c  or  50   d ) to thereby cause a negative current to flow through the load. A first IGBT ( 54   a ) is connected in series with each of the diodes ( 50   a   , 50   b ) and is rendered conductive and nonconductive at a frequency lower than the high-frequency signal. Also, a second IGBT ( 54   b ) is connected in series with each of the diodes ( 50   c  and  50   d ) and is rendered nonconductive when the IGBT and nonconductive at the lower frequency. When the first IGBT is rendered conductive, the second IGBT is rendered nonconductive, and vice versa. The inverters ( 38   a   , 38   b ) are controlled in synchronization with the firs t and second IGBTs ( 54   a   , 54   b ) in such a manner that the high-frequency signal can have a larger magnitude when the second IGBT ( 54   b ) is conductive than when the first IGBT ( 54   a ) is conductive.

[0001] This invention relates to a power supply apparatus for use inelectroplating, for supplying current to a load including an object tobe plated, an electrolytic solution and electrodes, to thereby plate theobject. (In this specification, this type of power supply apparatus isreferred to as electroplating power supply apparatus, and the load isreferred to as plating load.)

BACKGROUND OF THE INVENTION

[0002] In electroplating, it is known to invert, at a high speed, thepolarity of current supplied to a plating load. When current with thepositive polarity is being supplied to the plating load, plating takesplace, and when the current of the negative polarity is being suppliedto the load, the plating is interrupted or part of the metal forming aplated layer is dissolved back into the electrolyte solution, wherebycrystals forming the plated layer are made finer so that the object canbe uniformly plated.

[0003] When a multi-layered printed circuit board 2, like the one shownin FIG. 1, is plated, some problems have happened. The multi-layeredprinted circuit board 2 includes substrates 2 a and 2 b, for example, onwhich electronic components are integrated to a high density. Thecircuit board 2 are provided with a number of through-holes like athrough-hole 4, and a number of via-holes, like a via-hole 6. As thenumber of layered substrates is larger, the difference in thicknessbetween plated metal layers on an edge 4E and an inner wall 4IN of thethrough-hole 4 becomes larger, resulting in non-uniform plating. Inother words, it is difficult to uniformly plate the through-holes 4.Similarly, as the number of substrates increases, the difference inthickness between plated metal layers on an edge 6E and an inner wall6IN of the via-hole 6 becomes larger, which results in non-uniformplating. It has been found that in order to form a plated metal layeruniform in thickness over the entire surfaces of the substrates 2 a and2 b, it is necessary to make a negative-polarity plating current ofsufficiently larger magnitude flow for a shorter time period than apositive-polarity plating current.

[0004] In Japanese Patent Application No. HEI 10-281954 on Sep. 17, 1998(Japanese Patent Application Publication No. 2000-92841), inventorsincluding one of the inventors of the present invention proposed anelectroplating power supply apparatus which supplies current having apolarity inverted at intervals of, for example, from 5 to 20milliseconds, to a plating load, to thereby form a layer of uniformthickness over a plating load including a plurality of substrates, suchas a multi-layered printed circuit board. The apparatus of this Japanesepatent application is shown in FIG. 2.

[0005] The power supply apparatus shown in FIG. 2 includes DC powersupplies 10 a and 10 b, voltage-boosting converters 16 a and 16 b, andchoppers 22 a and 22 b, for supplying a plating load 24 with currentalternating between positive and negative polarities. Thevoltage-boosting converter 16 a includes a reactor 12 a and an IGBT 14a, and the voltage-boosting converter 16 b includes a reactor 12 b andan IGBT 14 b. The chopper 22 a includes a reverse current blocking diode18 a and an IGBT 20 a, while the chopper 22 b includes a reverse currentblocking diode 18 b and an IGBT 20 b. The IGBTs 14 a, 14 b, 20 a and 20b are controlled by a controller 26.

[0006] For example, when the IGBTs 20 a and 14 b are nonconductive andthe IGBTs 20 b and 14 a are conductive, current flows through the DCsupply 10 a, the reactor 12 a and the IGBT 14 a, resulting in storage ofenergy in the reactor 12 a. At the same time, a negative current issupplied to the plating load 24 from the DC supply 10 b, through thereactor 12 b, the diode 18 b and the IGBT 20 b.

[0007] Then, the IGBTs 20 a and 14 b are rendered conductive and theIGBTs 20 b and 14 a are rendered nonconductive, a positive current flowsfrom the DC supply 10 a through the reactor 12 a, the diode 18 a and theIGBT 20 a to the plating load 24 to plate an object to be plated. Inthis case, because of the turning off of the IGBT 14 a, the voltagebased on the energy stored in the reactor 12 a is superposed on thevoltage supplied by the DC power supply 10 a, resulting in rapid changeof the negative current flowing to the plating load 24 to the positivecurrent. At the same time, energy is stored in the reactor 12 b becausethe IGBT 14 b is conductive.

[0008] Then, the IGBTs 20 a and 14 b become nonconductive again, withthe IGBTs 20 b and 14 a rendered conductive, a negative current issupplied to the plating load 24 from the DC power supply 10 b throughthe reactor 12 b, the diode 18 b and the IGBT 20 b. In this case, too,the voltage generated in the reactor 12 b is superposed on the voltagesupplied by the DC power supply 10 b, so that the change from thepositive current to the negative current is rapid.

[0009] In this way, current with alternating polarity is supplied to theplating load 24, and a multi-layered printed circuit boards withthrough-holes and via-holes can be plated with a layer of a uniformthickness.

[0010] The power supply apparatus described above requires separate DCpower supplies 10 a and 10 b for positive and negative currents. Also,it requires, in addition to the IGBTs 20 a and 20 b used to switch themain current, the auxiliary IGBTs 14 a and 14 b for inverting the loadcurrent at a high speed, which makes the circuit arrangementcomplicated, which, in turn, leads to increase of the cost of theelectroplating power supply apparatuses.

[0011] In order to downsize the electroplating power supply apparatus,the DC power supplies 10 a and 10 b are downsized by arranging them torectify a commercial AC signal, convert the resulting rectificationoutput to a high-frequency signal in an inverter, and transform andrectify the high-frequency signal to a DC signal.

[0012] The value of commercial AC power supply voltage differ fromcountry to country or from area to area. Therefore, in electroplatingpower supply apparatuses for use in countries or areas where ACcommercial power supplies provide a “400 V group” voltage, i.e. avoltage of from 380 V to 460 V, the DC power supplies 10 a and 10 brequire an inverter including IGBTs which can withstand a voltageresulting from rectifying the “400 V group” AC voltage. However, suchIGBTs are not widely available, so they are expensive, leading toincrease of the cost of the power supply apparatuses.

[0013] An object of the present invention is to provide an inexpensivepower supply apparatus for use in electroplating, which can provideuniform electroplating.

SUMMARY OF THE INVENTION

[0014] An electroplating power supply apparatus according to the presentinvention includes an input-side rectifier for rectifying a commercialAC signal. The output signal of the input-side rectifier is converted toa high-frequency signal in a DC-to-high-frequency converter. A chopperor an inverter may be used as the DC-to-high-frequency converter. Aplurality of DC-to-high-frequency converters may be used, beingconnected in series. When a plurality of DC-to-high-frequency convertersare used, they are connected in series. The high-frequency signaloutputted by the DC-to-high-frequency converter is transformed in atransformer. The number of transformers is equal to the number of theDC-to-high-frequency converters. When plural DC-to-high-frequencyconverters are used, the same number of transformers are used beingconnected in parallel.

[0015] A first output-side rectifier is connected between thetransformer and a load to rectify a transformed high-frequency signalprovided from the transformer in such a way that a positive polaritycurrent can be supplied to the load when the transformed high-frequencysignal is positive in polarity. A second output-side rectifier connectedin parallel with the first output-side rectifier rectifies thetransformed high-frequency signal of negative polarity so that anegative polarity current can be supplied to the load. The first andsecond output-side rectifiers are arranged to perform full-wave orhalf-wave rectification.

[0016] A first semiconductor switching device is connected in serieswith the first output-side rectifier and is ON-OFF controlled by alower-frequency signal at a frequency lower than that of thehigh-frequency signal. A second semiconductor switching device isconnected in series with the second output-side rectifier and is placedin the opposite state to that of the first semiconductor switchingdevice in accordance with the lower-frequency signal. In other words,when the first semiconductor switching device is rendered conductive,the second semiconductor switching device is rendered nonconductive bythe lower-frequency signal, and vice versa.

[0017] The DC-to-high-frequency converter is so controlled insynchronization with the first and second semiconductor switchingdevices as to provide the high-frequency signal having a value largerduring a time when the second semiconductor switching device isconductive than during a time when the first semiconductor switchingdevice is conductive. The control of the DC-to-high-frequency convertermay be done by, for example, changing the value of a reference signalfor feedback control of the converter, in synchronization with thecontrol of the first and second semiconductor switching devices.

[0018] It is preferable that the period during which the secondsemiconductor switching device is conductive be shorter than the periodduring which the first semiconductor switching device is conductive.

[0019] With the above-described arrangement, when the firstsemiconductor switching device is rendered conductive with the secondsemiconductor switching device being nonconductive, positive current issupplied to the plating load from the first output-side rectifier. Onthe other hand, when the second semiconductor switching device isrendered conductive with the first semiconductor switching device beingnonconductive, negative current is supplied to the plating load from thesecond output-side rectifier. Since the DC-to-high-frequency converteris arranged to provide a larger high-frequency signal when the secondsemiconductor switching device is conductive, i.e. when the negativecurrent is being supplied to the plating load, than when the firstsemiconductor switching device is conductive, i.e. when the positivecurrent is being supplied to the plating load. Accordingly, the negativecurrent has a larger value so as to perform uniform plating.

[0020] The power supply apparatus with this arrangement requires onlyone input-side rectifier which functions as a DC supply. In addition,this electroplating power supply apparatus requires only twosemiconductor switching devices. Accordingly, the cost of the powersupply apparatus can be low.

[0021] First and second reactors may be connected to the first andsecond semiconductor switching devices, respectively. The first andsecond reactors are wound on the same core so that the positive voltageapplied to the plating load can increase when the second semiconductorswitching device is nonconductive, and the negative voltage applied tothe plating load can increase when the first semiconductor switchingdevice is nonconductive. For example, the first reactor may be wound onthe core in the direction opposite to the direction in which the secondreactor is wound.

[0022] When the first and second reactors are used in the manner asstated above, during a time when the first output-side rectifierrectifies the high-frequency signal from the transformer to providepositive current to the plating load, the first reactor dischargesadditional positive current to the plating load. Similarly, during atime when the second output-side rectifier rectifies the high-frequencysignal from the transformer to provide negative current to the platingload, the second reactor discharges additional negative current to theplating load. Accordingly, the conversion between the positive currentand the negative current supplied to the plating load can be performedat a high speed, which promotes the uniform plating.

[0023] Charge storage means may be charged when current is flowing inthe load. A capacitor may be used as the charge storage means, or asnubber circuit associated with the semiconductor switching device maybe arranged to function additionally as the charge storage means. Whenone of the first and second semiconductor switching devices is renderedconductive with the other being in the nonconductive state, dischargingmeans causes the charge storage means to discharge in such a manner thatdischarge current of the same polarity as the current currently flowinginto the plating load flows. The discharging means may be, for example,a switch connected between the charge storage means and the platingload.

[0024] With this arrangement, a charge stored in the charge storagemeans when current is supplied to the plating load is discharged whenthe polarity of the current flowing in the plating load is reversed. Thecharge is discharged in the polarity after the polarity reversal.Accordingly, the current to the plating load can be reversed from thepositive to negative polarity or from the negative to positive polarityso that the thickness of the resulting plated layer can be uniform.

[0025] The DC-to-high-frequency converter may include a convertingsemiconductor switching device and control means for ON-OFF controllingthe converting semiconductor switching device. The control meansprovides a control signal to ON-OFF control the converting semiconductorswitching device in such a manner that the difference of the positivecurrent flowing through the plating load from a positive currentreference value set for the positive current can become zero, and thedifference of the negative current flowing through the plating load froma negative current reference value set for the negative current canbecome zero. Sample and hold means is provided for sampling and holdingthe control signal provided by the converting semiconductor switchingdevice control means when the negative current flows into the platingload, and the sampled and held control signal is applied to theconverting semiconductor switching device when the current flowingthrough the load switches from the positive to negative current.

[0026] The reason for the use of the sample and hold means is asfollows. The DC-to-high-frequency converter is feedback controlled.However, when the current to the plating load is switched from, forexample, positive to negative polarity, the negative current cannotchange to the value corresponding to the negative current referencevalue simultaneously with the polarity switching since the positivecurrent reference value and the negative current reference value differfrom each other. To avoid this problem, the control signal produced whenthe negative current is supplied is sampled and held, and the sampledand held control signal is applied to the converting semiconductorswitching device of the DC-to-high-frequency converter when the currentis switched to the negative one so that the negative current can beinstantaneously changed to the one corresponding to the negative currentreference value. This results in uniform thickness of the plated layer.

[0027] Detecting means for detecting when the load is opened may be usedtogether with the first and second reactors. When the detecting meansdetects the open-circuiting of the load, that one of the first andsecond reactors through which current was flowing before such detectionis short-circuited by a short-circuiting semiconductor switching device.

[0028] Open-circuiting of the load for any reason causes the currentflowing currently through the first or second reactor becomes zero,which, in turn, causes a large voltage generated across the first orsecond reactor the current flowing through which has become zero.Application of such large voltage to the first or second semiconductorswitching device may cause damage to that semiconductor switchingdevice. To avoid it, when the load is open-circuited, the reactorthrough which current is flowing when the load is opened isshort-circuited to thereby prevent the voltage generated across it frombeing applied to the first or second semiconductor switching device, tothereby protect the semiconductor switching device.

[0029] The DC-to-high-frequency converter may be formed of two invertersconnected in series between the output terminals of the input-siderectifier. The two inverter configuration may be used when thecommercial AC power supply supplies an AC voltage to the input-siderectifier which outputs an output voltage twice the voltage eachinverter can bear.

[0030] With this arrangement, semiconductor devices which can withstanda lower voltage than the DC voltage produced from the input commercialAC voltage can be used as the semiconductor switching devices of theinverters. In other words, inexpensive semiconductor devices can beused, which, in turn, can reduce the cost of the power supply apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIG. 1 is a cross-sectional view of a multi-layered printedcircuit board;

[0032]FIG. 2 is a block circuit diagram of a prior art electroplatingpower supply apparatus;

[0033]FIG. 3 is a block circuit diagram of an electroplating powersupply apparatus according to an embodiment of the present invention;

[0034]FIG. 4 is a block circuit diagram of an inverter control unit ofthe electroplating power supply apparatus shown in FIG. 3; and

[0035]FIGS. 5A and 5B respectively show how load current of theelectroplating power supply apparatus shown in FIG. 3 changes and how areference signal used in the inverter control unit shown in FIG. 4changes.

DESCRIPTION OF PREFERRED EMBODIMENT

[0036] A power supply apparatus for use in electroplating according toone embodiment of the present invention is shown in FIG. 3. Theelectroplating power supply apparatus includes input terminals, forexample, terminals 30 a, 30 b and 30 c, adapted for connection to acommercial AC power supply, for example, a three-phase commercial ACpower supply. The commercial AC power supply provides, for example, a“200 V group” voltage, i.e. a voltage within a range of from 180 V to240 V, or a “400 V group” voltage, i.e. a voltage within a range of from380 V to 460 V.

[0037] The input terminals 30 a-30 c are connected to an input-siderectifier 32, which is a full-wave rectifier circuit includingrectifying diodes 32 a, 32 b, 32 c, 32 d, 32 e and 32 f. An output ofthe input-side rectifier 32 is connected through a parallel combinationof a thyristor 34 a and a resistor 34 b to a series combination of twosmoothing capacitors 36 a and 36 b.

[0038] DC-to-high-frequency converters formed of inverters 38 a and 38b, respectively, are connected in parallel with the smoothing capacitors36 a and 36 b, respectively. The inverters 38 a and 38 b can behalf-bridge type inverters. The inverter 38 a is formed of semiconductorswitching devices, e.g. MOSFETs, 40 a and 42 a, and capacitors 44 a and46 a, and the inverter 38 b is formed of semiconductor switchingdevices, e.g. MOSFETs, 40 b and 42 b, and capacitors 44 b and 46 b. Inplace of the half-bridge inverter, a full-bridge inverter may be used.

[0039] Each of the MOSFETs 40 a, 40 b, 42 a and 42 b is switched on andoff at a high frequency in response to an inverter control signalapplied to the gate electrode thereof from an inverter control unit 48,whereby a DC voltage produced across each of the smoothing capacitors 36a and 36 b is converted to a high-frequency voltage of, for example, 20KHz to 100 KHz. Flywheel diodes 49 a, 49 b, 49 c and 49 d are connectedin anti-parallel with the drain-source conduction paths of the MOSFETs40 a, 42 a, 40 b and 42 b, respectively.

[0040] High-frequency voltages from the inverters 38 a and 38 b areapplied across the primary windings 48 a-p and 48 b-p, respectively, oftransformers 48 a and 48 b, whose secondary windings 48 a-s and 48 b-sare connected in parallel. The secondary windings 48 a-s and 48 b-s haveintermediate taps 48 a-T and 48 b-T, respectively, which are connectedtogether.

[0041] A transformed high-frequency voltage developed across theparallel combination of the secondary windings 48 a-s and 48 b-s isrectified in an output-side rectifier 50, which includes diodes 50 a, 50b, 50 c and 50 d. The secondary windings 48 a-s and 48 b-s have upperones of their terminals of the secondary windings 48 a-s and 48 b-sconnected together to the junction of the anode of the diode 50 a andthe cathode of the diode 50 c, and have their lower terminals connectedtogether to the junction of the anode of the diode 50 b and the cathodeof the diode 50 d.

[0042] The outputs of the output-side rectifier 50 are connected toopposite ends of a series combination of semiconductor devices, forexample, IGBTs 54 a and 54 b, through smoothing reactors 52 a and 52 b,respectively. The reactors 52 a and 52 b include windings wound on thesame core, but in opposite directions. Therefore, when, for example,current flowing through the reactor 52 a to the IGBT 54 a stops, currentflows through the smoothing reactor 52 b toward the diodes 50 c and 50d. On the other hand, when current flowing through the reactor 52 btoward the diodes 50 c and 50 d stops, current flows through the reactor52 a toward the IGBT 54 a.

[0043] The IGBT 54 a and the reactor 52 a, and the IGBT 54 b and thereactor 52 b form choppers. The IGBTs 54 a and 54 b are ON-OFFcontrolled by a chopper control signal supplied from a chopper controlunit 56. The conduction and nonconduction periods of the IGBTs 54 a and54 b are determined through a timer 57 associated with the choppercontrol unit 56. The conduction period of the IGBT 54 a is set to belonger than the period during which the IGBT 54 b is conductive. Forexample, the period from the time the IGBT 54 b starts conducting to thetime it next starts conducting is about 5 milliseconds to about 20milliseconds.

[0044] The junction of the IGBTs 54 a and 54 b is connected to an outputterminal 58 a of the power supply apparatus. The other output terminal58 b is connected to the mutually connected intermediate taps 48 a-T and48 b-T on the secondary winding 48 a-s and 48 b-s of the transformers 48a and 48 b. The output terminals 58 a and 58 b are connected to aplating load 60.

[0045] When the IGBT 54 a is conductive, with the upper terminals of thesecondary windings 48 a-s and 48 b-s being more positive than themutually connected intermediate taps 48 a-T and 48 b-T, current flowsfrom the upper terminals through the diode 50 a into the load 60. On theother hand, if the lower terminals of the secondary windings 48 a-s and48 b-s are more positive than the intermediate taps 48 a-T and 48 b-T,current flows from the lower terminals through the diode 50 b to theload 60.

[0046] When the IGBT 54 b is conductive, with the upper terminals of thesecondary windings 48 a-s and 48 b-s being more negative than themutually connected intermediate taps 48 a-T and 48 b-T, current flowingfrom the intermediate taps 48 a-T and 48 b-T to the load 60 returns tothe upper terminals through the rectifying diode 50 c. If the lowerterminals of the secondary windings 48 a-s and 48 b-s are more negativethan the intermediate taps 48 a-T and 48 b-T, the current flowing fromthe intermediate taps 48 a-T and 48 b-T to the load 60 returns to thelower terminals through the rectifying diode 50 d.

[0047] Thus, the diodes 50 a and 50 b function together as a firstoutput-side rectifier, while the diodes 50 c and 50 d function togetheras a second output-side rectifier.

[0048] A positive current detector 62 a is connected between the reactor52 a and the IGBT 54 a for detecting the value of the current(hereinafter referred to as positive current) flowing toward the IGBT 54a. Similarly, a negative current detector 62 b is connected between thereactor 52 b and the IGBT 54 b for detecting the value of the current(hereinafter referred to as negative current) flowing toward the reactor52 b. The positive and negative current detectors 62 a and 62 b developpositive and negative current value representative signals,respectively, which are coupled to the inverter control unit 48.

[0049] As shown in FIG. 4, the inverter control unit 48 includes anerror amplifier 66, which develops an error signal representative of thedifference between the positive or negative current value representativesignal from the current detector 62 a or 62 b, and a reference signalprovided by a reference signal source 64. A PWM driver 68 provides aPWM-type inverter control signal to the inverters 38 a and 38 b forcontrolling the inverters in such a manner as to make the error signalbecome zero.

[0050] The reference signal from the reference signal source 64 is apulse signal as shown in FIG. 5B and has base portions B and peakportions P. The pulse signal is synchronized by a signal from thechopper control unit 56 with the turning-on and turning-off of the IGBTs54 a and 54 b. Specifically, when the positive current flows because ofthe conduction of the IGBT 54 a and the non-conduction of the IGBT 54 b,the base portion B of the pulse signal occurs. When the IGBT 54 a isnonconductive, the IGBT 54 b is conductive, and, therefore, the negativecurrent flows, the peak portion P occurs in the pulse signal. Since thepulse signal is synchronized with the turning on and off of the IGBTs 54a and 54 b, the period between the rising edge of one peak portion B tothe rising edge of the next adjacent peak portion P is, for example,from 5 milliseconds to 20 millisecond, which is significantly longerthan the period of the high-frequency signal from the inverters 38 a and38 b.

[0051] A sample and hold circuit 70 is provided in association with theerror amplifier 66. The sample and hold circuit 70 is responsive to thesignal from the chopper control unit 56 by sampling and holding theerror signal produced by the error amplifier 66 when the IGBTs 54 a and54 b are nonconductive and conductive, respectively, i.e. when the peakportion P occurs in the reference signal. When the IGBTs 54 and 54 b arenext rendered nonconductive and conductive, the sample and hold circuit70 provides the error signal which it holds to the PWM driver 68 inresponse to the signal from the chopper control unit 56.

[0052] In the circuit shown in FIG. 3, a snubber circuit 72 is connectedin parallel with a series combination of the collector-emitterconduction paths of the IGBTs 54 a and 54 b. The snubber circuit 72includes a series combination of a diode 74 and charge storage means,e.g. a capacitor 76 and absorbs an excessive voltage produced by thereactor 52 a or 52 b when IGBT 54 a or 54 b is rendered nonconductive.Between the junction of the diode 74 and the capacitor 76 and the outputterminal 58 b, a series combination of a resistor 78, thecollector-emitter conduction of a semiconductor switching device, e.g.an IGBT 80, and a reverse current blocking diode 82 is connected. Whenan ON signal is applied to the gate of the IGBT 80 from the choppercontrol unit 56, the IGBT 80 becomes conductive, and a charge on thecapacitor 76 is discharged to the output terminal 58 b.

[0053] The collector-emitter conduction path of an IGBT 84 a, operatingas a semiconductor switching device, is connected between the junctionof the positive current detector 62 a and the collector of the IGBT 54 aand the output terminal 58 b. Similarly, the emitter of an IGBT 84 b,operating as a semiconductor switching device, is connected to thejunction of the negative current detector 62 b and the emitter of theIGBT 54 b, with the collector of the IGBT 84 b connected to the outputterminal 58 b. When the IGBTs 84 a and 84 b receive at the respectivebases an ON signal from the chopper control unit 56, they becomeconductive to thereby connect the reactors 52 a and 52 b to the outputterminal 58 b, respectively. When a voltage detector 86 connectedbetween the output terminals 58 a and 58 b detects a zero voltagebetween the output terminals 58 a and 58 b, the chopper control unit 56develops the ON signal.

[0054] The operation of the electroplating power supply apparatus withthe above-described arrangement is described with reference to FIGS. 5Aand 5B. When a commercial AC power supply is connected to the inputterminals 30 a-30 c of the power supply apparatus at a time t1 shown inFIG. 5A, the thyristor 34 a is open, and, therefore, the output of theinput-side rectifier 32 is supplied through the resistor 34 b to thecapacitors 36 a and 36 b, resulting in charging of the capacitors 36 aand 36 b. When the charging is completed, the thyristor 34 a is renderedconductive, and after that, the output of the input-side rectifier 32 issupplied through the thyristor 34 a to the smoothing capacitors 36 a and36 b, so that the rectifier output is smoothed into a DC voltage.

[0055] The DC voltages across the smoothing capacitors 36 a and 36 b areapplied to the inverters 38 a and 38 b, respectively, where they areconverted into high-frequency voltages, which, in turn, are applied tothe primary windings 48 a-p and 48 b-p of the transformers 48 a and 48b. Transformed high-frequency voltages are induced in the secondarywindings 48 a-s and 48 b-s.

[0056] In this state, if the IGBTs 54 a and 54 b are rendered conductiveand nonconductive, respectively, by the chopper control unit 56 insynchronization with the reference signal provided by the invertercontrol unit 48, the value of the high-frequency voltages induced in thesecondary windings 48 a-s and 48 b-s are smaller because, in this state,it is the base portion B (FIG. 5B) that occurs in the reference signal.

[0057] In this state, if the voltages induced at the upper terminals ofthe secondary windings 48 a-s and 48 b-s are higher than the voltages atthe intermediate terminals 48 a-T and 48 b-T, a smaller, positivecurrent shown in FIG. 5A flows through the diode 50 a, the reactor 52 a,the positive current detector 62 a, the IGBT 54 a, the output terminal58 a, the plating load 60, the output terminal 58 b to the intermediateterminals 48 a-T and 48 b-T.

[0058] If voltages higher than the voltages at the intermediateterminals 48 a-T and 48 b-T are present at the lower terminals of thesecondary windings 48 a-s and 48 b-s, a smaller, positive current, shownin FIG. 5A, flows through the diode 50 b, the reactor 52 a, the positivecurrent detector 62 a, the IGBT 54 a, the output terminal 58 a, theplating load 60, the output terminal 58 b to the intermediate terminals48 a-T and 48 b-T.

[0059] The positive current is detected by the positive current detector62 a, which provides a positive current value representative signal tothe inverter control unit 48. The inverter control unit 48 controls theconduction periods of the MOSFETs 40 a, 40 b, 42 a and 42 b in such amanner that the positive current can have a value corresponding to thebase portion B of the reference signal (FIG. 5B).

[0060] Because of the flow of the positive current, the capacitor 76 ofthe snubber circuit 72 is also charged.

[0061] At a time t2 shown in FIG. 5A, the chopper control unit 56renders the IGBT 54 a nonconductive and IGBT 54 b conductive. Thereference signal from the inverter control unit 48 changes to the peakportion P shown in FIG. 5B, the high-frequency voltages induced in thesecondary windings 48 a-s and 48 b-s have a larger value.

[0062] In this state, if the voltages at the intermediate taps 48 a-Tand 48 b-T on the secondary windings 48 a-s and 48 b-s are higher thanthe upper terminals of the secondary windings, a negative current flowsfrom the mutually connected intermediate terminals 48 a-T and 48 b-Tthrough the output terminal 58 b, the plating load 60, the outputterminal 58 a, the IGBT 54 b, the negative current detector 62 b, thesmoothing reactor 52 b, and the diode 50 c to the mutually connectedupper terminals of the secondary windings 48 a-s and 48 b-s.

[0063] If the voltage at the mutually connected intermediate taps 48 a-Tand 48 b-T is higher than the voltage of the mutually connected lowerterminals of the secondary windings 48 a-s and 48 b-s, a negativecurrent flow from the intermediate taps 48 a-T and 48 b-T through theoutput terminal 58 b, the plating load 60, the output terminal 58 a, theIGBT 54 b, the negative current detector 62 b, the smoothing reactor 52b and the diode 50 d to the mutually connected lower terminals of thesecondary windings 48 a-s and 48 b-s. The negative current is detectedby the negative current detector 62 b, and the negative current valuerepresentative signal is applied from the detector 62 b to the invertercontrol unit 48. Then, the inverter control unit 48 controls theconduction periods of the MOSFETs 40 a, 42 a, 40 b and 42 b of theinverters 38 a and 38 b in such a manner that the detected negativecurrent assumes the value corresponding to the peak portion P of thereference signal.

[0064] Due to the switching from the positive current to the negativecurrent, the current flowing through the plating load 60 changes fromthe smaller, positive current to the larger, negative current, as shownin FIG. 5A. No current flowing through the smoothing reactor 52 a,through which the positive current has been flowing, induces in thesmoothing reactor 52 b, a current which tends to flow into the diodes 50c and 50 d, which adds to the negative current. This accelerates thechange from the positive current to the negative current.

[0065] At time t2, the chopper control unit 56 renders the IGBT 80conductive, whereby a positive charge on the capacitor 76 flows throughthe output terminal 58 b, the plating load 60, the output terminal 58 a,the IGBT 54 b, the negative current detector 62 b, the smoothing reactor52 b, and the diode 50 c or 50 d. Then, the negative current increases,accordingly. This further accelerates the positive to negative currentchange. The negative current melts the plated layer on the edges of thethrough-holes and via-holes, so that uniform electroplating can berealized.

[0066] At time t3 in FIG. 5A, the IGBTs 54 a and 54 b are renderedconductive and nonconductive, respectively, again, so that the positivecurrent can flow as described previously, and the inversion from thenegative to positive current takes place. The period during which thenegative current flows is shorter than the period during which thepositive current flows, and the negative current is larger than thepositive current. Due to the positive current starting to flow, thenegative current which has been flowing through the reactor 52 b ceases,while the positive current starts to flow through the reactor 52 atoward the IGBT 54 a. This negative to positive current change takesplace rapidly. At the same time, the capacitor 76 is charged.

[0067] In a similar way, positive and negative currents are alternatelysupplied to the plating load 60. Due to repetitive supply of a negativecurrent having a larger value for a shorter period than a positivecurrent to the plating load 60, the object to be plated in the platingload 60 can be plated with a layer of uniform thickness. When thepositive current flow changes to the negative current flow and when thenegative current flow changes to the positive current flow, a negativecurrent and a positive current induced respectively in the smoothingreactors 52 b and 52 a are superposed, which accelerates the inversionof the current flow. In addition, when the positive current changes tothe negative current, current based on the charge on the capacitor 76 isadded to the negative current in a sense to increase the negativecurrent. Accordingly, the speed of the inversion from the positive tonegative current is further increased.

[0068] With the arrangement described above, the difference in valuebetween the positive current and the negative current is large.Therefore, unless the response of the inverters 38 a and 38 b are fast,it would take a long time for the current to attain the desired negativevalue, which would result in insufficient melting of the plated layer onthe object. It is, therefore, necessary for the inverters 38 a and 38 bto have an increased response. For that purpose, the sample and holdcircuit 70 in the inverter control unit 48 shown in FIG. 4 operates tosample and hold the error signal developed when the negative current isbeing supplied, and supplies the error signal it has held to the PWMdriver 68 when a command for the positive to negative current inversionis developed. With this arrangement, the response of the inverters 38 aand 38 b can be increased so that the high-frequency voltages developedin the inverters 38 a and 38 b can be converted to high voltages forsupplying the negative current without delay.

[0069] The plating load 60 includes, in addition to the object to beplated, a hanger for handing the object to be plated, and other things.Sometimes, it may occur that the contact between the hanger and theobject is broken, resulting in open-circuiting of the plating load 60.This would cause zero current to flow through the positive currentdetector 62 a and the negative current detector 62 b, which, in turn,causes the inverters 38 a and 38 b to be controlled in such a ways thata maximum voltage can be developed between the output terminals 58 a and58 b. At the same time, large voltages would be induced in the reactors52 a and 52 b because the currents which have been flowing therethroughno longer flow. This would cause excessive voltages to be applied to theIGBTs 54 a and 54 b, which may damage the IGBTs 54 a and 54 b. Inaddition, if, under this situation, the open-circuiting of the platingload 60 disappears, excessive current would flow into the plating load60.

[0070] In order to avoid it, the voltage detector 86 is arranged todetect an excessive voltage caused by the open-circuiting of the platingload 60. When the voltage detector 86 detects such excessive voltage, itdevelops a signal indicating the development of such excessive voltage,which is applied to the chopper control unit 56. In response to it, thechopper control unit 56 renders conductive that one of the IGBTs 84 aand 84 b which is connected to the reactor 52 a or 52 b through whichthe current has been flown, to thereby short-circuit that reactor. Thisprevents an excessive voltage from being applied to the IGBTs 54 a and54 b, and also prevents excessive current from being supplied to theplating load 60 when the open-circuiting disappears.

[0071] Since the inverters 38 a and 38 b are connected in series, it isnot necessary to use, as the MOSFETs 40 a, 40 b, 42 a and 42 b, MOSFETswhich can withstand a voltage resulting from rectifying a 400 V groupcommercial AC voltage even when the power supply apparatus is intendedfor use with a commercial AC power supply supplying a 400 V groupvoltage, but they need to withstand only a voltage resulting fromrectifying a 200 V group AC voltage. Needless to say, the power supplyapparatus can be used with a 200 V group commercial AC power supply,too.

[0072] The described power supply apparatus includes two inverters,namely the inverters 38 a and 38 b, so that it can be used with not onlya commercial AC power supply supplying a 200 V group AC voltage but alsowith a commercial AC power supply supplying a 400 V group AC voltage.However, a power supply apparatus for use with only a 200 V groupcommercial AC power supply needs only one inverter. In this case, thenumber of the transformers may be reduced to one, with only two diodesused for the output-side rectifier, namely, the diodes 50 a and 50 b inthe circuit shown in FIG. 3.

[0073] In the above-described embodiment, the current detectors 62 a and62 b are connected in series with the reactors 52 a and 52 b,respectively, but they may be connected between the intermediate taps 48a-T 48 b-T and the output terminal 58 b, respectively.

[0074] In the above-described embodiment, the capacitor 76 is arrangedto be discharged only when the positive current changes to the negativecurrent. However, another diode equivalent to the diode 74 may be used,with its cathode connected to the junction of the capacitor 76 and thediode 74 and with its anode connected to the output terminal 58 b, sothat the capacitor 76 can be charged when the negative current is beingsupplied, too. In this case, a series combination of a resistor, an IGBTand a reverse current blocking diode, equivalent to the seriesconnection of the resistor 78, the IGBT 80 and the reverse currentblocking diode 82, is additionally connected between the junction of thecapacitor 76 and the diode 74 and the output terminal 58 a. When theIGBT 80 is rendered conductive, the IGBT in the equivalent seriescombination is also made conductive so that the charge on the capacitor76 can be supplied also when the current changes from negative topositive, which increases the speed of the negative to positive currentchange, too. It should be noted that it is not always necessary to useboth current change acceleration arrangements, but only one of them,namely, the arrangement shown in FIG. 3 for causing the capacitor 76 todischarge only when the current changes from positive to negative, orthe added arrangement described just above for causing the capacitor 76to discharge only when the current changes from negative to positive,may be employed.

[0075] Instead of the voltage detector 86, a current detector may beused for detecting open-circuiting of the plating load 60.

[0076] A chopper may be used in place of an inverter as theDC-to-high-frequency converter.

[0077] Further, in place of IGBTs 54 a and 54 b, MOSFETs or bipolartransistors may be used. Also, in place of MOSFETs, IGBTs or bipolartransistors may be used to form the inverters 38 a and 38 b.

What is claimed is:
 1. A power supply apparatus for use inelectroplating, comprising: an input-side rectifier adapted forrectifying commercial AC power; a DC-to-high-frequency converter forconverting an output of said input-side rectifier to a high-frequencysignal; a transformer for transforming a high-frequency signal from saidDC-to-high-frequency converter and developing a high-frequencytransformer output signal; a first output-side rectifier connectedbetween said transformer and a load in such a manner as to rectify saidhigh-frequency transformer output signal of said transformer to cause apositive current to flow in said load when said high-frequencytransformer output signal is of positive polarity; a second output-siderectifier connected in parallel with said first output-side rectifierfor rectifying said high-frequency transformer output signal of saidtransformer, when said high-frequency transformer output signal is ofnegative polarity, to cause a negative current to flow in said load; afirst semiconductor switching device connected in series with said firstoutput-side rectifier and rendered conductive and nonconductive at afrequency lower than said high-frequency signal; and a secondsemiconductor switching device connected in series with said secondoutput-side rectifier, said second semiconductor switching device beingnonconductive when said first semiconductor switching device isconductive, and conductive when said first semiconductor switchingdevice is nonconductive; said DC-to-high-frequency converter beingcontrolled in synchronization with said first and second semiconductorswitching devices in such a manner that said high-frequency signal has alarger magnitude when said second semiconductor switching device isconductive than when said first semiconductor switching device isconductive.
 2. The power supply apparatus according to claim 1 furthercomprising: first and second reactors connected respectively to saidfirst and second semiconductor switching devices; wherein said first andsecond reactors are wound on a same core in such a manner that when saidsecond semiconductor switching device is nonconductive, a positivevoltage to be applied to said load can be increased, and when said firstsemiconductor switching device is nonconductive, a negative voltage tobe applied to said load can be increased.
 3. The power supply apparatusaccording to claim 1 further comprising: charge storage means chargedwhen current is being applied to said load; and discharging meansresponsive to turning on of one of said first and second semiconductorswitching devices for discharging said charge storage means in such amanner that discharge current of the same polarity as the currentflowing through said load can flow.
 4. The power supply apparatusaccording to claim 1 wherein said DC-to-high-frequency converterincludes a converting semiconductor switching device and control meansfor turning said converting semiconductor switching device on and off,said control means providing a control signal for ON-OFF controllingsaid converting semiconductor switching device in such a manner that adifference between a positive current flowing through said load and apositive-current reference value set for the positive current can becomezero and that a difference between a negative current flowing throughsaid load and a negative-current reference value set for the negativecurrent can become zero; wherein said power supply apparatus comprisesfurther sample-and-hold means for sampling and holding said controlsignal provided by said control means when said negative current flowsthrough said load, and for supplying the sampled and held control signalto said converting semiconductor switching device when the currentflowing through said load changes from a positive current to a negativecurrent.
 5. The power supply apparatus according to claim 3 furthercomprising: detecting for detecting when said load is open-circuited;and short-circuiting semiconductor switching device for short-circuitingthat one of said first and second reactors through which current isflowing when said detecting means detects the open-circuiting of saidload.
 6. The power supply apparatus according to claim 1 wherein saidDC-to-high-frequency converter comprises two inverters connected inseries between output terminals of said input-side rectifier, and saidinput-side rectifier is to be connected to such a commercial AC powersupply that said input-side rectifier can provide a rectified outputvoltage which is about two times as large as the voltage which each ofsaid inverters can handle.